Plumbing manifolds are control centers for hot and cold water that feed supply lines to individual fixtures. Separated manifold chambers or separate manifolds can serve hot and cold water lines. The cold water manifold is fed from the main water supply line and hot water manifold is fed from the water heater. A water line dedicated to each fixture employs a port in the manifold.
Important requirements in hot and cold water domestic plumbing systems are:    1. High resistance to temperature aging for water temperatures up to 200° F.    2. High resistance to stress cracking.    3. High resistance to chemical solvents (water additives, antifreeze solutions).    4. Lowest possible linear thermal expansion.    5. High tensile strength.    6. High form stability.    7. High resistance to abrasion.    8. High resistance to deformation.    9. Dimensional tube tolerances.    10. Internal and external tube wall smoothness.    11. Behavior during long term internal pressure creep test which takes into account the temperature-dependent aging behavior of the pipe material at water temperatures up to 200° F.To combat this premature aging the molecules within the PE are realigned in order to give greater stability to the material itself. The cross linking process takes place within the molecular structure of the plastic material. The most common thermoplastic materials currently being used for heating and plumbing pipe, often referred to as polyolefin materials are: Polyethylene (PE), Polypropylene (PP), and Polybutelene (PB) (generic term for polybutylene). Among this family of polyolefin plastics, only Polyethylene has been determined to have the molecular structure which lends itself perfectly to the cross linking process.
“Un-cross-linked” polyethylene (PE) as it is formed receives its basic dimension and wall thickness, is composed of long hydrocarbon string molecules forming a loosely held together array of hydrogen and carbon atoms which can be compared to a beaded curtain swaying in a breeze. This is basically the molecular composition of the PE which is available at hardware stores and is suitable only for non-critical applications. A material in this form is not suitable for heating and plumbing applications. Within a relatively short period of time the material fatigues under the stress of water temperature and pressure as well as temperature cycling and the beaded curtain of molecules splits open without resistance. By cross-linking the molecular “beads” (hydrocarbon string molecules) and forming cross-connections which are referred to as cross-linking bridges, the string molecules form a three dimensional network of hydrocarbon molecules. The “beaded curtain” becomes transformed into a fishing net with strength and stability.
In this way, the previously non-applicable PE is transformed, after cross-linking, into a completely different material with all the desired characteristics demanded for heating or plumbing. After the cross-linking of the PE, its molecular mobility is severely impeded by the cross-linking bridges between the string molecules. The material does not flow or melt and its form becomes stable against heat. The material holds it shape at all temperatures, even exposed to blow torch temperatures until it chars or burns. The thermoplastic has been transformed into a thermoset material by cross-linking, eliminating the melting point or liquid phase of the material.